A number of stringed musical instruments have a ``nonlinear sound''
that comes from modulating the physical termination of the string (as
opposed to its acoustic length in the case of tension modulation).

The Finnish Kantele [232,515] has a different effective
string-length in the vertical and horizontal vibration planes due to a
loose knot attaching the string to a metal bar. There is also
nonlinear feeding of the second harmonic due to a nonrigid tuning peg.

Perhaps a better known example is the Indian sitar, in which a
curved ``jawari'' (functioning as a nonlinear bridge) serves to
shorten the string gradually as it displaces toward the bridge.

The Indian tambura also employs a threadperpendicular
to the strings a short distance from the bridge, which serves to
shorten the string whenever string displacement toward the bridge
exceeds a certain distance.

Finally, the slap bass playing technique for bass guitars
involves hitting the string hard enough to cause it to beat against
the neck during vibration
[265,369].

In all of these cases, the string length is physically modulated in
some manner each period, at least when the amplitude is sufficiently
large.